In a normal overhead cam motor, there is one cam lobe per valve. The cam spins which moves a rocker arm up and down which controls the movement of the valve to which it is attatched. In a motor with 4 valves per cylinder, the primary lobe is attatched to one valve and the secondary is attatched to the other.

In a VTEC motor, there is an extra cam lobe and rocker arm between the primary and secondary rockers; this is called the mid lobe and rocker arm and is not actually attatched to valves. Before VTEC is engaged, the mid rocker arm freewheels. It has a part called a "lost motion assembly" under it which acts similar to a valve spring.

When VTEC is engaged, an oil pressure switch opens and little pins inside the mid rocker arm lock it to the primary and secondary rockers, thus changing the effective cam profile.

If you have a cam with stock primary and secondary lobes, it will behave identical to stock before VTEC is engaged. The only difference is that once VTEC kicks in you get a more aggressive cam profile than the stock mid lobe provides. If you have larger primary and secondary lobes, you get a more agressive cam profile all the time.